Margaret Bramall, who has died aged 90, was for two crucial decades the public face and guiding spirit of a body we now know as One Parent Families - first established in 1918 by Lettice Fisher as the National Council for the Unmarried Mother and her Child. Under her direction from 1962 until 1979, the council transformed itself from a welfare agency for destitute young mothers to a high-profile organisation with the political nous to campaign in the public arena to improve the lot of all families headed by a lone parent.

With the reserve of her generation, Margaret did not parade her personal experience of the problems she worked to alleviate for others. But she was herself a lone parent of two young sons, both with health problems. Her former husband was Ashley Bramall (obituary, February 12, 1999), knighted in 1975 for a career in Labour politics, education and local government that included leading the Inner London Education Authority and chairing the Greater London Council.

They had met at Oxford. Margaret Taylor, as she then was, came up from St Paul's girls' school, London, to read history at Somerville. People saw her and Ashley as a flamboyant pair, prominent in the university Labour club. Like many of her contemporaries, Margaret also joined the Communist party for a time. She was ready to back any good cause. She demonstrated against fascism in London and helped feed the Jarrow marchers as they passed through Oxford.

When a Somervillian was sent down after being found in bed with a man in her lodgings (the man was only rusticated for a term), Margaret supported a plan to get every woman who had slept with a man to sign a petition to Miss Darbishire, the august principal, to rescind her decision - or send them all down too. They were counting on 30 fellow criminals. But no one else would sign. Margaret married Ashley after their graduation in 1939. She later studied social science at the London School of Economics and acquired a further qualification at the Institute of Almoners.

After her divorce, apart from her devoted care of her sons, her work at the National Council became her life. "She felt she was the council, which was both a strength and a weakness," a colleague recalls. If she had a fault, it was that she found it hard to delegate, but her leadership skills were considerable and she showed perspicacity in appointing the Jamaican-born Pauline Crabbe, who proved a fine deputy. Margaret's style was warm, humane and hands on. "She didn't seem to care much what she looked like, but she attracted people," recalled a sometime colleague.

When she began at the council the plight of unmarried mothers and their children was freighted with prejudice and punitive morality. If society helped the woman instead of condemning her, the belief ran, it would thereby condone immorality. The destitution and stigma of illegitimacy borne by these families were seen as sad consequences of moral laxity (for female sinners only), flowing from divine justice rather than social injustice.

Margaret cut through this miasma and her outrage at its cruelty and unfairness was the driving force of her work. No doubt her private pain and anger, though well hidden from her colleagues, sharpened her campaigning edge and fuelled the energy, courage and tenacity of her commitment to what remained a deeply unpopular cause.

She was a mover and shaper of attitudes; intelligent, articulate, intimidated by nobody. Genuine empathy characterised her dealings with women, whose stories she would tell council members with passionate concern. But she realised that their primary task no longer lay in answering the women's welfare needs, which the state was finally taking on. She began to think in terms of influencing and formulating policy, and to talk in these terms to government, local authorities, and politicians of all parties.

A quantum leap took place during her two decades' service with the council. It expanded its remit to include all one-parent families, changing its title in 1973. "Our council dislikes the pejorative words 'unmarried mother' and 'illegitimate child' and does not use them unless it seems essential," Margaret declared. She fought for the dignity of all lone parents, whatever their gender or marital status, and for changes in the laws affecting children born out of wedlock. In the long term, she won. "She wanted one-parent families to be treated as a normal part of society," said Judge Stephen Lloyd, who chaired the council at the end of her tenure.

Yet Margaret's proudest achievement eventually proved a deep disappointment. The Labour government's appointment of a royal commission under Sir Morris Finer in 1969 to consider the problems of one-parent families was both a triumph and a watershed for the council. It held a conference, produced a report and sent 280 recommendations to the Finer committee. The Finer report of 1974 made 230 recommendations, most of them the council's. The Observer hailed it as one of the major social documents of the century. Margaret predicted that it would revolutionise the lives of most one-parent families. But she added with foreboding that if it were shelved, "a tremendous opportunity to help over one million children will be lost". And it was lost.

Margaret retired from the council in some sadness in 1979, though she then lectured in social studies at Surrey University for a further decade. In her last years, her greatest pleasures were in her garden at Twickenham, and her family.

There was always an aura around Margaret. If you were in a room full of people you would notice her, and she had a gift for mobilising support from influential well-wishers and sympathetic journalists which served her well at the council.

She is survived by her two sons, seven grandchildren and five great-grandchildren.

· Margaret Elaine Bramall, social campaigner, born October 1 1916; died August 11 2007